In general, such electrochemical sensors, in the case of which the relative change in the equilibrium Galvani voltage between a measured medium and a sensing electrode is preferably effected by the activity change of predominantly a certain kind of ion, are referred to as ion-selective electrodes. Such ion-selective electrodes allow relatively simple and fast determining of ion concentrations in different media, e.g. also in turbid and colored solutions. Ion-selective electrodes are applied, for example, in process-liquid analysis or in waste water analysis.
Potentiometric measurements with ion-selective electrodes metrologically correspond largely to classic pH measurements technology on the basis of pH glass membrane electrodes. Referencing a reference potential of a reference electrode with an essentially constant potential, e.g. the known Ag/AgCl electrode, the ion concentration in a measured medium can, by means of a high-impedance voltmeter, be determined with high accuracy with little apparatus-related effort.
Besides glass membranes, so-called solid body or polymer membranes are also currently used as ion-selective components of such electrodes. The latter comprise, as a rule, a so-called plasticizer as a lipophilic solvent, a salt of the kind of ion to be measured with a lipophilic counter ion, and a polymer material as a network former for solidification of the membrane. Especially in the case of cation-selective membranes, an ionophore is frequently also present. FIG. 1 schematically shows the basic construction of such a polymer membrane electrode. Ion selective electrodes of this manner of construction are described, for example, in “Ion-selective Electrodes”, J. Koryta and K. Stulik, Cambridge University press, 1983, S. 61, or in “Das Arbeiten mit lonenselektiven Elektroden (Working with Ion-Selective Electrodes)”, K. Cammann, H. Galster, Springer, 1996.
The ion-selective electrode 1 includes a tubular housing 2, which is sealed on one end with a closure cap 3, and on the other end by an ion-selective polymer membrane 5, i.e. a polymer membrane including a selectivity-providing component. In the housing interior 7 surrounded by the housing 1, a sensing system is located, which leads the electrical potential of the membrane off to an electrically conductive contact or connection cable 9. The sensing system comprises an inner electrolyte 11 and a sensing electrode 13, which, for example, can be formed of a silver wire coated with silver chloride. The inner electrolyte 11 usually contains a salt of the ion to be determined, as well as additional chloride ions for stabilizing the electrical potential difference between the inner electrolyte 11 and sensing electrode 13. The polymer membrane 5 usually has a thickness of less than 1 mm and is composed as previously described.
With its membrane bearing end, the ion-selective electrode 1 can be immersed in a measured medium, e.g. in a liquid, in which the ions to be detected are dissolved, in order to determine the concentration of a certain kind of ion in this medium. In the case of the electrode construction shown in FIG. 1, in measurement operation, the polymer membrane 5 has contact with the medium across its entire base.
In the case of use of the ion-selective electrode for continuous measured value registering in liquid analysis, the period of use achievable under real conditions is of great importance. As a rule, the ion-selective components and plasticizer of the polymer membrane have low, but nevertheless noticeable, water solubility. This leads to the washing out or bleeding of the selectivity-providing component or of the plasticizer, and therewith to a change in the composition of the polymer membrane, which, in turn, effects a change in the membrane properties. After a period of use of some months in measurement operation, the ion-selective electrode, or the membrane, must therefore be replaced. The average period of use of an ion-selective polymer membrane electrode is, in addition to the solubility of the membrane components in the measured medium, also influenced by the thickness or the volume of the membrane. Also the flow velocity of the measured medium, and in given cases surfactants or organic solvent present in the measured medium, can increase the solubility of the membrane components in the measured medium. The washing out of membrane components occurs especially intensively in online measurements technology, in the case of which the ion-selective membrane is continually located in a liquid flow.
The consequences of the washing out of membrane components are, among other things,                drift of the calibration parameters, slope and zero-point;        change in the Nernst concentration range, i.e. the range in which a linear relationship between the logarithm of the ion concentration to be measured and the measured voltage exists;        change in the limit of detection;        change in the selectivity of the electrode;        change of the isothermal point of intersection;        lengthening of the response time.        
In WO 93/21520 A1, measures are described for lengthening the period of use of a polymer membrane electrode. These include the saturation or over-saturation of the inner electrolyte and, as far as is possible, also of the measured medium, with the components of the membrane tending toward being washed out. Described in an example of an embodiment is the introduction of a conditioning cartridge into the housing interior of the ion-selective electrode. The cartridge contains all important membrane components tending toward being washed out are present, adsorbed on a highly disperse solid phase with a large specific surface. This highly disperse solid phase is in contact with an aqueous electrolyte solution in the housing interior, so that this is practically continually saturated or over-saturated with the membrane components tending toward bleeding out, and a resupplying of these components to the membrane is achieved. Furthermore, a difficultly soluble salt of the ion to be determined should be placed directly in the membrane. Via the named measures, the period of use of the ion-selective electrode in the case of uninterrupted contact with a measured medium should be increased.
A disadvantage of an ion-selective electrode embodied in such a manner is the fact that the measured medium can be saturated or over-saturated with the membrane components tending toward bleeding only in exceptional cases. In WO 93/21520 A1, it is provided that the measured medium be conducted through a cartridge, in which all important membrane components tending toward being washed out are present, adsorbed on a highly disperse solid, phase with large specific surface. Since in the course of a longer time of use, large volumes of measured medium flow through such an apparatus, it must be assumed that the cartridge will be fouled and clogged by solid and dissolved sample components, or lose its usefulness by other means, for example, through microbial growth.